“ If they did, presumably they would have found a way to avoid the Holocaust and still keep everyone Frum.”
Not sure if you know, but Reb Elchanan was very well aware of the impending Holocaust with its full impact. He spoke about it. He cried about it. It was very vivid to him. I know of other Baalei Ruach Hakodesh who were very well aware of it. One Tzaddik spoke openly about ‘rivers of tears’ before anything began, and tried everything possible to expand Torah so as to avoid the coming Gezeira. The forces working against Torah were stronger.
I know how Ruach Hakodesh and prophesy are related, but what does Ruach Hakodesh or prophesy have to do with omnipotence?